THERE are two different stories about what happened at Veletta Heron's kitchen table one winter's evening.

Mrs Heron told a Sydney court this week it was there that her grandson Joshua Staples felt safe enough to admit lighting several bushfires.

Mr Staples, 20, had been working as a Rural Fire Service volunteer when he allegedly lit 10 scrub and rubbish fires in the Badgerys Creek area in January 2011.

Grandmother and witness ... Veletta Heron. Photo: Tamara Dean

Mrs Heron and Mr Staples were going over legal paperwork at her Orchard Hills home when she said she realised the case against her beloved grandson was strong.

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''Josh was going in and out of the street and within seconds of him coming out the fire started there, and that's why I kept saying 'Well, why were you down there?'''

Then 18, Mr Staples explained he had driven down the streets to think, Mrs Heron said.

''You don't go down there and a fire starts the moment you stop thinking and drive out,'' she told Downing Centre Local Court.

Mrs Heron said she also asked him a question fearing she knew the answer: did he burn down a family caravan and torch his sister's car?

She said Mr Staples broke down and admitted everything.

''I said to Josh: 'You know how much we've lost from that caravan? There were my mother's things in there and my father's things and things we can never replace.'

''I said, 'What about the car?' and he said, 'I've never had a car given to me or anything given to me.'''

But, from her grandson's point of view, at the other end of the dining table on that day in mid-2011 something very different was happening.

Mr Staples, who said he had an accident in high school that left him temporarily paralysed and unable to deal with stress, felt the pressure of his grandmother's questions weighing on him.

''She just kept telling me it was a strong circumstantial case and if it was looked at by a magistrate or judge it would be determined I was guilty and I needed to plea guilty,'' Mr Staples said.

He said he was ''quite upset and stressed … because she just kept asking me questions and when I'd give her an answer, she didn't like the answer''.

He denied he made any admissions to his grandmother.

The court heard Mr Staples pleaded guilty to the charges but changed his plea to not guilty in November 2011, which strained the relationship between his mother and grandmother.

Mr Staples's barrister, Michael Coroneos, said Mrs Heron, who at one point looked across the courtroom at her grandson and cried, saying she could not lie for him, had put on a performance while in the witness box.

''She tried to play the sympathy card; she did the whole act of crying on cue,'' Mr Coroneos said.

But it was Mr Staples who was the unconvincing witness, said the police prosecutor, Sergeant Daniel McMahon.

''When the family unit started to fall apart … over the course of these proceedings … you decided to stay with your mother and hang your grandmother out to dry,'' Sergeant McMahon said.

''What do you mean?'' replied Mr Staples.

The magistrate, Michelle Goodwin, said she found Mrs Heron's evidence reliable and would not exclude it.